The present invention relates to the fabrication of semiconductor-based devices. More particularly, the present invention relates to improved techniques for maintaining the transparency of observation windows employed in plasma-enhanced semiconductor processing systems.
In the fabrication of semiconductor-based devices, e.g., integrated circuits or flat panel displays, layers of materials may alternately be deposited onto and etched from a substrate surface. As is well known in the art, the etching of the deposited layers may be accomplished by a variety of techniques, including plasma-enhanced etching. In plasma-enhanced etching, the actual etching of the substrate (e.g., the semiconductor wafer or the glass panel) typically takes place inside a plasma processing chamber. To form the desired pattern on the substrate's layer(s), an appropriate mask, which may be either a photoresist or a hard mask, is typically provided. A plasma is then formed from a suitable etchant source gas to etch areas of the substrate that are unprotected by the mask, leaving behind the desired pattern.
Among different types of plasma processing systems, inductively coupled plasma processing systems have proven to be highly suitable for forming the ever shrinking features on the substrate. In general, an inductively coupled plasma processing system employs a powered electrode to maintain the etching plasma by inductively coupling with it through a dielectric window.
To facilitate discussion, FIG. 1 depicts a simplified sketch of an exemplary plasma processing system typical of the type employed to etch substrates during the manufacture of semiconductor-based devices. In FIG. 1, the plasma processing system depicted happens to be a low pressure, high density inductively-coupled plasma processing system. As will be apparent to those skilled in the art, however, the invention is not so limited.
Referring now to FIG. 1, a plasma processing system 100 includes a plasma processing chamber 102. Above chamber 102, there is disposed an electrode 104 which is implemented by a coil in the example of FIG. 1, although other mechanisms for inductively coupling the RF energy to the plasma within the plasma processing chamber may also be employed. Electrode 104 is energized by a radio frequency (RF) generator 106 via a matching network (conventional and omitted to simplify the illustration). In the example of FIG. 1, RF generator 106 supplies RF energy having a frequency of about 13.56 MHz although other appropriate frequencies may also be employed.
Within plasma processing chamber 102, there is shown a dielectric window 110, representing the dielectric window through which the plasma inside the chamber is inductively coupled with electrode 104. In the typical case, dielectric window 110 is formed of a suitable dielectric material, such as fused silica or alumina, that permits the aforementioned inductive coupling action to take place. Etchant source gases may be introduced into the chamber interior by an appropriate gas distribution apparatus (not shown to simplify the illustration) such as a shower head, or a gas distribution ring, or simply ports disposed near dielectric window 110 or in the chamber walls as is shown. Through the inductive coupling action, an etching plasma is formed in the RF-induced plasma region 112 between dielectric window 110 and a substrate 114.
Substrate 114 having thereon the layer(s) to be etched is introduced into plasma processing chamber 102 and disposed on a chuck 116, which acts as a second electrode and is preferably biased by a radio frequency generator 118 through a matching network (conventional and omitted to simplify the illustration). For the purpose of the present invention, chuck 116 may represent any type of workpiece holder arrangement, such as an electrostatic chuck, or a mechanical clamping-type chuck, and may assume any configuration, including a cantilever configuration. Like RF generator 106, RF generator 118 in the example of FIG. 1 may source RF energy having any suitable frequency (e.g., between 2-13 MHz in some systems). There is provided an exhaust port 120, which may be coupled to an appropriate turbo pump arrangement to facilitate the removal of the etch byproducts and to maintain the desired low pressure within chamber 102 during etching.
To avoid etching too deeply into substrate 114, which may unintentionally damage the underlying regions, it is important to terminate the etch at the appropriate time. There exist in the art many techniques for ascertaining when etching should terminate, some of which require monitoring the progress of the etch in order to ascertain the end point. In certain low density plasma etch reactors, for example, interferometric techniques have been employed to ascertain the depth of the etch at a given point in time to ascertain whether etching should be permitted to continue. An example of an interferometry-based etch depth monitoring technique may be found in U.S. Pat. No. 5,450,205.
In general, interferometric techniques of etch depth monitoring involve illuminating the area being etched with an illumination source, which may be a laser light source, a tungsten/halogen lamp, plasma emission, or the like, and monitoring the reflected beam for the characteristic periodic optical interference pattern of repetitive maxima and minima. Using an appropriate logic circuit, information pertaining to the current depth of the etch may be ascertained from the pattern of repetitive maxima and minima in the reflected beams. Such information may then be employed, for example, to end point the etch. Interferometric techniques of etch depth monitoring are well known in the art and will not be discussed in detail for brevity's sake.
In a typical plasma processing chamber, both the light source and the optoelectronic device employed to monitor the reflected beams are typically disposed outside of the chamber to avoid exposing these components to the corrosive etching environment within the plasma processing chamber. Because of this, the plasma processing chamber needs to have a relatively transparent observation window through which the illuminating beams and the reflected beams may pass. An exemplary arrangement is depicted in FIG. 2 wherein both an illuminating beam 202 from a light source 204 and a reflected beam 206 pass through transparent observation window 212. For completeness, collimating and focusing optics arrangement 208 as well as detector 210 are also shown.
It has been observed, however, that the performance of the interferometry-based monitoring technique tends to degrade significantly when the transparency of the observation window degrades. The loss of transparency may occur when the interior surface of the observation window is roughened due to the deposition of etch byproducts. The observation window may also lose some of its transparency when its interior surface is etched by the etching plasma within the plasma processing chamber. At some point, the increasing opacity of the observation window degrades the signal-to-noise ratio of the reflected beam to the point where useful information pertaining to the etch depth may no longer be obtained. In some cases, the loss of transparency occurs as quickly as during the etch of a single substrate. This results in a need to either clean the chamber or replace the observation window, either of which increases the costs associated with processing substrates.
In view of the foregoing, there are desired improved techniques for prolonging the time during which the observation window remains transparent and useful for interferometry-based monitoring purposes.